IPT systems are now widely used in industry and elsewhere to couple power from one reference frame to another without physical contact. An example of such a system is described in specification U.S. Pat. No. 5,293,308, the contents of which are incorporated herein by reference.
IPT technology allows large amounts of electrical energy to be transferred between two loosely coupled inductors over relatively large air gaps. An IPT system can be divided into two sections—a primary supply and one or multiple secondary pickups. The, or each, pickup receives power inductively from the primary. For an IPT system used in material handling applications, multiple secondary pickups are coupled on one long track as shown in FIG. 1, and the coupling coefficient between the primary and secondary inductors is typically around 0.01-0.1. In order to transfer large amounts of power (>1 kW) to each secondary, the primary supply generates a current in the range of 10-80 A and a frequency in the order of 10-40 kHz to overcome the low coupling conditions. Currently, IPT applications have been used in a wide variety of industrial and commercial applications.
In order to improve power transfer capacity in the IPT system, some compensation or tuning capacitor is required in the secondary pickup. The two most common compensation topologies used in the pickup are parallel and series tuned systems as shown in FIG. 1. Parallel tuning gives a constant current source property and series tuning gives a constant voltage source property. For the series tuned pickup, the voltage source property is ideal for driving most common types of loads. However, it is difficult to exactly match the induced voltage of the pickup to the desired output voltage as the tolerance in the inductor windings can easily create a 10% deviation in the output voltage. This 10% error may not be acceptable for many commercial or industrial loads. As such, a switch mode controller is usually required after the pickup to regulate the output voltage to its desired value with a minimal amount of error.
One technique is to use primary side control to achieve voltage regulation on the secondary pickup. This method sends feedback signals such as output voltage of the secondary pickup back to the primary converter via a wireless communication channel. Generally, primary side control has two possible methods of realization—frequency control or primary current control.
For applications such as material handling systems with multiple secondary pickups, control on the primary side cannot be used since regulating voltage on one pickup will affect the operation of other pickups which may be operating at different power levels. One conventional method to regulate the output voltage on the secondary side is to use a linear voltage regulator after the pickup. However, due to the tolerance of the output voltage of the pickup and the poor efficiency of the linear regulator, this topology is limited to low power applications. Another method cascades a buck converter after the series tuned pickup to regulate the output voltage with more electrical efficiency. However, this is not ideal because of the large number of components required which increase cost. In addition, the two stage (AC-DC and DC-DC) conversion process has losses in each stage which reduce efficiency. Other secondary side control techniques directly regulate power on the AC side to deliberately tune or detune the resonant tank circuit by adding extra reactance. One technique to realize a variable reactance component is to use a magnetic amplifier to produce a variable inductor. Although this may vary the AC power directly, the use of a variable inductor in the non-linear region of the B-H curve can limit the efficiency of the overall system. In addition, the variable inductor is expensive to manufacture because it has to manage the high resonant current without fully saturating.